While internally geared rear hubs aren't very common in the world of mountain biking, they still pop up from time to time. Oftentimes, you'll find an internally geared rear hub coupled with a belt drive system.

Much like the gears on a motorcycle or a car, the gearing on a hub such as this is completely internal, and consequently is not exposed to the elements like a regular drivetrain. However, they are generally much heavier, and sometimes much more expensive, than a standard drivetrain.